The present invention generally relates to the use of Code Division Multiple Access (CDMA) communications techniques in cellular radio telephone communication systems, and more particularly, to a receiver for jointly demodulating a plurality of CDMA signals with multipath time dispersion.
CDMA or spread spectrum communications has been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications. Examples include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in cellular systems is to change from analog to digital transmission; equally important is the choice of an effective digital transmission scheme for implementing the next generation cellular technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by cellular carriers using the next generation digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slob is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the available time slots as well as by limitations imposed by channel reuse.
With FDMA or TDMA systems or hybrid FDMA/TDMA systems, the goal is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both time and frequency. Thus, all CDMA signals share the same frequency spectrum and in the frequency or time domain, the CDMA signals appear to overlap one another.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be up to twenty times that of existing analog technology as a result of the properties of a wide band CDMA system, such as improved coding gain/modulation density, voice activity gating, sectorization and reuse of the same spectrum in every cell. CDMA transmission of voice by a high bit rate decoder ensures superior, realistic voice quality. CDMA also provides for variable data rates allowing many different grades of voice quality to be offered. The scrambled signal format of CDMA eliminates cross talk and makes it very difficult and costly to eavesdrop or track calls, ensuring greater privacy for callers and greater immunity from air time fraud.
In a CDMA system, each signal is transmitted using spread spectrum techniques. In principle, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or -1. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
A plurality of spread information signals modulate a radio frequency carrier, for example by binary phase shift keying (BPSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and despread. If quadrature phase shift keying (QPSK) modulation is used, then the signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate two carriers at the same frequency, but ninety degrees different in phase.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or -1, sometimes denoted "0" or "1". The signature sequence usually comprises N bits, and each bit is called a "chip". The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The traditional receiver correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. Only the real part of the correlation value is computed. When a large positive correlation results, a "0" is detected; when a large negative correlation results, a "1" is detected.
The "information bits" referred to above can also be coded bits, where the code used is a block or convolutional code. Also, the signature sequence can be much longer than a single transmitted symbol, in which case a subsequence of the signature sequence is used to spread the information bit.
In many radio communication systems, the received signal includes two components: an I (in-phase) component and a Q (quadrature) component. This occurs because the transmitted signal has two components (e.g. QPSK), and/or the intervening channel or lack of coherent carrier reference causes the transmitted signal to be divided into I and Q components. In a typical receiver using digital signal processing, the received I and Q component signals are sampled and stored at least every T.sub.c seconds, where T.sub.c is the duration of a chip.
In mobile communication systems, signals transmitted between two locations typically suffer from echo distortion or multipath rime dispersion, caused by, for example, signal reflections from large buildings or nearby mountain ranges. Multipath dispersion occurs when a signal proceeds to the receiver along not one but many paths so that the receiver receives many echoes having different and randomly varying delays and amplitudes. Thus, when multipath rime dispersion is present in a CDMA system, the receiver receives a composite signal of multiple versions of the transmitted symbol that have propagated along different paths (referred to as "rays") having relative time delays of less than one symbol period. Each distinguishable "ray" has a certain relative time of arrival m T.sub.c seconds and spans N of the I and Q chip samples, since each signal image is an N-chip sequence. It is typical to let m=0 correspond to the arrival of the earliest signal ray. Also, each ray has a certain amplitude and phase, which is represented by a complex channel coefficient c(m).
As a result of multipath time dispersion, the correlator outputs several smaller spikes rather than one large spike. To detect the transmitted symbol (and recover the information bit), the spikes received are combined. Typically, this is done by a RAKE receiver, which is so named because it "rakes" all the multipath contributions together.
A typical form of the RAKE receiver is shown in FIG. 1. A received radio signal is demodulated, for example, by mixing it with cosine and sine waveforms and filtering the signal in an RF receiver 100. Also, the resulting I and Q signals are sampled and quantized, yielding I and Q chip samples, which can be viewed as complex samples whose real part is the I sample and whose imaginary part is the Q sample. These complex samples are passed serially through a correlator 101, which correlates the complex received samples to the conjugate of the known signature sequence, producing a series of complex correlations.
A gating function 103 determines which correlation values are to be used for detection. The gating function 103 passes correlation values to a half complex multiplier (HCM) 104 at such times as are determined by the timing control unit 102. At those times, which occur once per ray per transmitted symbol, the gating function 103 passes a correlation value to the half complex multiplier 104, which multiplies the complex correlation with the appropriate RAKE tap coefficient, computing only the real part of the product. The RAKE tap coefficients are the conjugates of channel tap estimates provided by the channel tracking unit 105, which uses correlation values from the correlator 101 to estimate the channel tap locations (m values) and coefficients (c(m) values). Accumulator 106 accumulates the outputs of the HCM 104 and sends the final sum, once per transmitted symbol, to a threshold device 107. The threshold device 107 detects a binary "0" if the input is greater than a threshold, or a binary "1" if the input is less than the threshold. The threshold is typically zero.
The detailed operation of a conventional correlator 101 is shown in FIG. 2. The complex chip samples from the RF receiver are sent to an internal tapped delay line 200. There is also a tapped buffer 201, which stores the known signature sequence or its complex conjugate. For each chip sample that is fed into the tapped delay line 200, a correlation is performed with the conjugate of the signature sequence. Complex multipliers 202 multiply received samples with conjugated signature sequence values. The resulting complex products are summed in the complex summer 203. The resulting correlation value is sent to the gating function 103 as shown in FIG. 1.
The conventional RAKE receiver gives good performance provided several conditions are satisfied. The first condition is that the autocorrelation function of the signature sequence is ideal, in that the signature sequence is uncorrelated with a shift of itself. If this is not true, then the different signal rays interfere with one another, which is referred to as self interference. The second condition is that the crosscorrelation between the signature sequence of the desired signal and various shifted versions of the signature sequences of the other CDMA signals is zero. If this is not true, then the other CDMA signals interfere with the desired CDMA signal, degrading performance. This can be a particularly bad problem when another CDMA signal has a much higher power than the desired CDMA signal, referred to as the near-far problem. The third condition is that the interference caused by an echo of one transmitted symbol overlapping with the next transmitted symbol be negligible. If this is not true, then transmitted symbols interfere with past and future transmitted symbols, which is commonly referred to as intersymbol interference (ISI).
The theory of the design of good signature sequence sets indicates that there are fundamental limitations which prevent the first two conditions from being satisfied simultaneously. Consequently, performance will be limited by self interference, other signal interference, and ISI. There has been work to address the problem of other signal interference in an environment that does not experience multipath time dispersion. This is referred to as joint demodulation with no multipath. Note, for example, S. Verdu, "Minimum Probability of Error For Asynchronous Gaussian Multiple-Access Channels," IEEE Trans. Info. Theory, Vol. IT-32, pp. 85-96, R. Lupas and S. Verdu, "Linear multiuser detectors for synchronous code-division multiple-access channels," IEEE Trans. Inform. Theory, Vol. 35, pp. 123-136, Jan. 1989; and R. Lupas and S. Verdu, "Near-far resistance of multiuser detectors in asynchronous channels," IEEE Trans. Commun., Vol. 38, pp. 496-508, Apr. 1990. In these works, two approaches for jointly demodulating multiple CDMA signals are presented.
The first approach, known as Maximum Likelihood Sequence Estimation (without multipath), determines the most likely set of transmitted information bits for a plurality of CDMA signals without multipath time dispersion. This is illustrated in FIG. 3. As in the RAKE receiver, an RF receiver 300 receives a plurality of CDMA radio signals which are filtered and mixed down to I and Q baseband waveforms, which are sampled and quantized yielding complex received data values. These are sent to a plurality of correlators 301, each of which correlates to a particular signature sequence. However, because the channel is assumed not to have multipath time dispersion, only one correlation is kept per transmitted symbol. The one selected for use in demodulation is determined by the timing control unit 302, which signals the gating functions 303 to pass a value once per transmitted symbol period. Because the different CDMA signals may not be time aligned, i.e. asynchronous, the gates do not necessarily close at the same time. The correlation values are passed to a decision algorithm unit 304, which uses this information to decide the transmitted bit sequences of each CDMA channel. The algorithm used is the Viterbi algorithm, which determines the most likely information bit sequences. Note, however, that this receiver does not allow for a plurality of signal rays, i.e., by only passing one correlation value per transmitted symbol period from the gating function, and does not track channel coefficients.
The second approach, known as the decorrelation receiver, decorrelates the different CDMA signals so that they no longer interfere with one another. The method follows the same approach shown in FIG. 3. The only difference between these first and second approaches is the decision algorithm used, which is illustrated in FIG. 4. Correlation values from the gating functions, block 303 in FIG. 3, are stored in a buffer 400. When the buffer is full, the set of correlation values are viewed as a vector of values. This vector is multiplied by a decorrelation matrix in the matrix multiplier 401. The matrix used for the multiplication consists of signature sequence crosscorrelation values. The product of the matrix and the vector is a vector of decorrelated detection statistics, one for each CDMA signal, which are fed to thresholding devices 402. These produce the detected information bit values.
Both of these approaches do not address the problem of multipath time dispersion. Thus, they give approaches for the joint demodulation of CDMA signals in the absence of multipath time dispersion.
Recently, Wijayasuriya and others have proposed a form of the decorrelation receiver intended for multipath time dispersion. See S. S. H. Wijayasuriya, G. H. Norton, and J. P. McGeehan, "A near-far resistant sliding window decorrelating algorithm for multi-user detectors in DSCDMA systems," Proc. Globecorn '92, Orlando, Fla., pp. 1331-1338, Dec. 1992, S. S. H. Wijayasuriya, J. P. McGeehan, and G. H. Norton, "Rake decorrelating receiver for DS-CDMA mobile radio networks," Electronics Letters, vol. 29, no. 4, pp. 395-396, 18 Feb. 1993, and S. S. H. Wijayasuriya, J. P. McGeeban, and G. H. Norton, "RAKE decorrelation as an alternative to rapid power control in DS-CDMA mobile radio," 43rd IEEE Vehicular Technology Conference, Secaucus, N.J., pp. 368-371, May 18-20, 1993. In these papers, decorrelation is applied to decorrelate multiple rays of each signal. However, these rays are combined noncoherently. Thus, there is no channel estimation and performance is limited to that of noncoherent methods.
Also, Zvonar and Brady have proposed both an MLSE and a decorrelation receiver for joint demodulation in multipath. For the MLSE receiver, see Z. Zvonar and D. Brady, "Optimum detection in asynchronous multiple-access multipath Rayleigh fading," Twenty-sixth Annual Conf. on Information Sciences and Systems, Princeton, University, March 1992. In this receiver, correlations with the full signature sequence are made at the different ray arrival times. These correlations are RAKE combined to give a combined signal for each user, which are then passed to a decision algorithm. This has the disadvantage in that the combined signals depend on future as well as past transmitted symbols, which increases the complexity of the decision algorithm.
For the decorrelation receiver, see Z. Zvonar and D. Brady, "Coherent and differentially coherent multiuser detectors for asynchronous CDMA frequency-selective channels," Milcom '92, San Diego, Calif., Oct. 11-14, 1992, and Z. Zvonar and D. Brady, "Suboptimum multiuser detector for synchronous CDMA frequency-selective Rayleigh fading channels," Communication Theory Mini-Conference, Orlando, Fla., Dec. 6-9, 1992. In the first article, the decorrelation is implemented assuming an "infinite-horizon" detector, which leads to a matrix FIR filter, followed by a bank of IIR filters. This implementation has two disadvantages: 1) the filtering is noncausal, so that future data values are required, and 2) the effort required to determine the filter coefficients can be substantial, as a matrix inverse is required. In the second article only synchronous signals are considered, intersymbol interference is ignored, and decorrelation is performed as a matrix inverse. This has the disadvantage of not addressing asynchronous signals and not addressing intersymbol interference. Also, the matrix inversion may be quite costly, especially when the channel changes with time. Finally, both of these articles do not separate the channel effects (which may be time varying) from the signature sequence correlation effects (which change when signals are stopped or new signals are started).
Thus, the growing demand for radio communications raises the need to optimize the performance and capacity of wireless communications systems. CDMA technology can give high system capacity. To maximize CDMA capacity in a mobile radio environment, other signal interference, self interference, and ISI must be minimized in an efficient manner. Existing approaches fail to address all of these problems.